Photon counting applications have decisive impact on advances in a wide range of fields, from astronomy and metallurgy to environmental protection, health diagnostic, medical analysis and imaging.
In detectors based on photon counting, the detection of each photon is typically performed as follows:
The photon interacts with a specific scintillator that converts its energy into a short electrical signal (the conversion can be done in one stage or in multiple stages).
The electrical signal, with a specific duration in time is then amplified, shaped and compared with a specific threshold.
Every signal that has an amplitude that is higher than the threshold, switch “on” a comparator. The comparator in turn, outputs a digital signal with a duration equal to the time in which the input signal was higher than the specific threshold.
A counter that counts the pulses at the output of the comparator outputs the rate of the photons that interact with the scintillator.
This method endures some disadvantages while the major difficulty is in pile-up phenomena that occurs when two or more photons are detected as a single event. In such case, the comparator sees both photons as one. The phenomena of detection circuit being blocked for detection of a subsequent event while handling a previous event is also called dead time. Pile up or dead time phenomena are aggravated in cases in which the time interval between photon events behaves according to Poisson statistics that are the typical cases of interest.
Other electronics schemes can be used for single photon detection, however with similar shortcomings.
One option that is used in order to resolve pile-up is to work at low count rates, a solution that limits the possibilities of photon counting.
The other option is to perform pile-up corrections. Pile-up corrections are usually made in prior art methods during the pre-amplifier stage where the signal is shaped or optimized before its comparison to the threshold value. An example for a system that discloses treatment in the preamplifier zone is U.S. Pat. No. 5,952,662 “High event rate gamma camera” by McDaniel filed in 1997. The disclosed apparatus is for use with gamma camera to increase the count rate without causing “dead time” or pile up. The disclosed apparatus includes a first processor optimized for simplicity and minimum dead time but with moderate or poor spatial resolution for generally determining the impact point of a photon on a scintillator crystal and a second digital processor that uses the general position information to identify a subset of PMT intensity signals for further processing to identify the precise impact point location. In another example disclosed by Dirkse et al. in U.S. Pat. No. 4,535,242 “Method and a circuit for processing pulse of a pulse train”, tails of pulses are being eliminated. To eliminate the tails of an “A” pulse, for example, a first signal is produced at the occurrence of a “T” pulse. This first signal simulates the tail of the “A” pulse and it is subtracted from the tail pile up “T” pulse. The problem in these methods is that overdose is not directly detected.
It is a long felt need to provide a reliable photon counting-based detector provided with pile-up correction means in which overdose is detected without limitation to the counter value.
The scope of the present invention is to provide a method to remove/resolve the ambiguity, enabling by this accurate pile-up correction at very high count rates.